Be sure you have the correct lights installed and not the ggeneric wal-mart lights.
Brake light lamp P21/5W-12V/5W Turn signal lamp P21W-12V 21W
Prolonged use of the incorrect wattage bulbs could have a detrimental effect on the socket strip.
Bring the brake lamp and turn signal lamp from the left side.
If that corrects the problem then install the correct lamps.
Try bringing the entire socket strip from the left side snd right to left.
If the problem moves to the left side then you probably have a defective socket strip.
If the problem persists on the right side then your problem could be either in the connector or the wireing harness.

Ross.
Your problem is a broken or disconected eath wire on the rear lights. The earth wire is brown in colour and runs from the plug on the lighs to a stud under the floor. Should be easy to find the brake by inspecting both ends of the wire. Eric

Ross.
Your problem is a broken or disconected earth wire on the rear lights. The earth wire is brown in colour and runs from the plug on the lighs to a stud under the floor. Should be easy to find the brake by inspecting both ends of the wire. Eric

I answered over on the other thread. Good description Eric. Nice to include wire color.

What if an incorrect single contact lamp were installed which would bridge the double contact in the socket? The pins on the lamp base should prevent installation, but you never know.... vic

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Ross.
Your problem is a broken or disconected eath wire on the rear lights. The earth wire is brown in colour and runs from the plug on the lighs to a stud under the floor. Should be easy to find the brake by inspecting both ends of the wire. Eric

Sorry for not getting back on in a few days, been out.

Thanks for the reply's... to clarify

I need to follow the brown earth wire from the rear lighting fixture to where it grounds to the frame somewhere?

I need to follow the brown earth wire from the rear lighting fixture to where it grounds to the frame somewhere?

You need to buy a tool (isn't that wonderful!)... a cheap "multimeter" (Volt-Ohm-Amps).
Then remove all of the bulbs from the fixture, have NO lights turned on, and measure the ohms from the shell (not the pins) of the Brake light socket to some bare metal attached to the vehicle's frame. You should see zero (i.e. very very low) ohms.
Anything (say) 5 ohms or higher is an indication of a bad ground
(the brown wire could be frayed, which would still give a low ohm reading, but wouldn't pass enough current when the bulb was lit)
If it reads zero (or very close to), then the next test is to put the bulbs back in, set the meter to Volts (at least 12), and measure the shell to frame Voltage when the Brake lightbulb is lit.
Now the voltage should be zero (assuming no turn signal is lit). If you see significant (more than 2 volts) voltage (and the turn signal is lit), then that's a strong indicator of the faulty brown wire.
If it were me, and i saw that, i would simply run a NEW brown wire to the frame, and ignore the old one.

Quote:

Do i need to pull up the flooring to do this?

Probably not... the cable harness probably exits through the metal in the pillar area, and the actual attachment to the frame is under the rear.

You either replaced the bulb with the wrong one or installed it wrong.
You have a floating ground on the brake so that when you put on the brakes, it feeds through to the turn light.
Maybe you didn't get it put back together completely.
There are some reports that the metal light frame can rust and make bad connections.

Any chance you have trailer wiring on your vehicle?
If you do, check all of what the others have suggested, but also use the multimeter
to check the diodes in the trailer wiring.
When the blocking diodes that prevent the current from flowing from the brake and turn signal
wiring in your Sprinter (individual bulbs) gets sent to a trailer wiring harness (multi filament bulbs) it can tie the brake lights and turn signals together (bad blocking diode) and perhaps cause the condition you are seeing.
The remedy is to replace the diode block. Not a fun task, but if you have a bad diode, this is about the only thing that will fix the problem.
Roger

Ross.
The brown wire goes into a loom covered with a cloth tape, the loom goes down to a gromit in the vertical panel under the floor. look under the back and you will see the loom going across between the spare and the rear panel, you will see 2 brown wires coming out of the loom and going to a stud welded to the frame. take the nut off this stud [10mm] and look at the lug and the studd to see if they are clean. Eric.

sorry it's been so long to reply. I have solved the issue. Thank you very much for all the help. It was the trailer wiring harness. It was causing it to loop. I simply cut out the old one and wired in a new one.